The required application rates of ammonia in pounds per acre are quite varied depending on the crop, rainfall, the quality of the soil, the previous crop, the type of seed, etc. In general, the more vegetation above the soil the greater the requirement for ammonia. Applicator knife spacing is generally greater for corn, sorghum and the larger grains than it is for wheat, rice and the smaller grains. Some crops are sensitive to nitrogen rate, for example, popcorn and rice are not very tolerant of over or under application, and therefore the distribution across the tool bar from the manifold to the applicator knives is very important.
Ammonia at one atmosphere has a dew point of -28.degree. with a latent head of 598.3 BTU and is stored as a liquid in a pressurized container under pressure due to its own vapor pressure. Any drop in pressure of the system requires a related temperature drop. The temperature drop is provided by the vaporizing of liquid within the system.
The behavior of ammonia in a system applying it to the soil is very similar to the capillary control of a refrigerating system, where resistance to flow is thermal as well as physical. In the application of ammonia, it is desirable to overcome the thermal resistance of flow physically with throttling means within the meter. The thermal resistance to flow can be expressed as the reduction of mass per unit volume. The ideal manifold would be one that presents to each of the discharges a product of equal mass per unit volume and of equal velocity. At very low rates of application, the liquid and vapor will separate with the liquid seeking the inner surfaces of the manifold receiving less outside head. The usual manifold having some plugged outlets behaves very similar to the vapor degreaser only at a much lower temperature. Should there be three or more orificed outlets grouped with plugged outlets on either side, the refrigeration due to the pressure drop across the orificed outlets will provide more mass to the center outlet and this condition will perpetuate itself due to the temperature drop across the orifices.
Conventional manifolds presently in use have a fairly large, disc shaped, central interior with an inlet at the top. Better manifolds have a screen separating the inlet from the discharges, such as manifold No. A60075, manufactured by Continental NH.sub.3 Products Co. of Dallas, Tex. John Blue Co. manufactures an adjustable orificed 24 outlet manifold, its manifold No. A-6600.